The design of a half-drill has been used in the aircraft sheet metal and arms manufacturing industries, among other industrial applications for many years. The half-drill was often used to hold a tight tolerance of a drill hole for precision industrial drilling. The concept of a half-drill tip design is not known to have ever been used before for use in living bone.
A half-drill, such as that used in the sheet metal industry, is shown in FIG. 1 and is comprised of an elongated cylindrical shaft 1a having a cutting tip 9a at the distal end of the shaft 1a. The cutting tip 9a has a semi-circular cross-section. One-half side of the cylindrical shaft is milled away at the distal end of the shaft to produce the semi-cylindrical tip. This provides a longitudinally flat surface 2a along a center plane of the drill. The cutting tip 9a is further prepared by providing a taper 3a which produces two angled edges 4a' and 4a at the distal end of the flat surface 2a, and which produces a radial relief on the non-cutting edge. The two edges 4a' and 4a come together at a center point 5a.
FIG. 2 illustrates another embodiment of a prior art half-drill which utilizes a pilot tip 8a at the distal end of the cutting tip 9a'. The pilot tip 8a has a smaller diameter than the proximal end of the cutting tip 9a'. The proximal end of the tip 9a' has a diameter equal to the diameter of the shaft 1a.
Drills are used in orthopaedic surgery to drill holes in bone to prepare a channel for bone screws which are used for the fixation of bone or with bone screws used in conjunction with bone plates for the fixation and stabilization of bone. Typically, a standard twist drill is used for this type of routine drilling.
There are a number of disadvantages to using standard twist drills for direct drilling of bone. A standard helical, twist drill must be started perpendicular to the bone's surface to prevent its slipping out of position. Also, when a twist drill is just ready to proceed through the wall of cortical bone, the drill "grabs", takes a large forward motion and requires a great increase in torque to continue rotation. This irregular rough action can fracture the bone or can cause excessive unanticipated and uncontrolled penetration into soft tissue. This is a danger particularly to nerves and blood vessels.
Helical or twist drills of different sizes, cutting angles, rake angles and material compositions have been used in orthopaedic surgery for many decades.
A variety of bone pins are also commonly used in orthopaedic surgery with external skeletal immobilization devices or alone. In cases where bone pins are used with external fixation devices, the bone pins are drilled directly through the flesh and into the bone, and then left in place to be attached to an external immobilization device. One reason why twist drill tips are not used on these bone pins is because problems would be encountered with the potential for damaging nerves and blood vessels which could potentially become trapped and wound-up by a helical twist drill.
Fixation pins and wires standardly used today often have three or four facet trochar tips or spade style tips. A three facet trocar tip 15 and a spade style tip 14 are shown as examples of prior art in FIG. 3. Another style tip 16 shown in FIG. 3 will be referred to as an arrowhead tip. The biggest disadvantage of these pins, as well as other known styles of bone pins, is the excessive amount of heat generated, which can cause thermal necrosis or death of tissue due to heat. Larry S. Matthews, M.D. and Carl Hirsh, M.D., Ph.d. have written an article entitled "Temperatures Measured in Human Cortical Bone When Drilling," copyrighted 1972 by The Journal of Bone and Joint Surgery, Vol. 54-A, No. 2, pp. 297-308, March, 1972. This article notes other references indicating that thermal necrosis due to drilling has been reported, and that the relationship of thermal necrosis to a loss of stability in fixation is established. The article goes on to discuss that thermal damage to living tissue is related to the magnitude of temperature elevation and the period of time during which the tissue is subjected to damaging temperatures. Heat generated when drilling is due to friction and to the fragmentation of particles of bone at the cutting edge of the drill. It is to be noted that bone pins having a tip such as the spade, arrowhead or trochar tip do not allow for the extrusion of bone chips as the drilling progresses. Therefore, with these types of tips the bone chips are compressed into the hole wall causing further mechanical damage. Although a twist drill style tip allows for chip removal, as previously stated, the twist drill tip is not suitable for many types of bone drilling for external fixation. Twist drills also do not present the thermal problem that the current bone pins present, yet twist drills are not suitable for drilling through living flesh. It is also noted that much greater temperature elevations are noted when a worn drill is used.